The invention relates to the field of frequency-tunable ultra-high frequency filters with variable capacitance tuning devices.
In general terms transmission systems and particularly telecommunications systems are designed to operate in a given frequency band which may have several channels and the ultra-high frequency filters of the system must be tuned to the desired channel. When the system is intended to operate on the fixed frequency channel the filters can be adjusted in the factory or during their installation in a final manner. However, when the system is intended to successively operate on several frequency channels the so-called "frequency-mobile" filters must be able to rapidly and simply pass from one channel to another. In all cases it is necessary to provide tuning means for the ultra-high frequency filters used. The smaller the number of components to be varied and the less their setting influences the characteristics of the filter, other than the tuning frequency, the easier said tuning can be performed.
At present a number of different types of ultra-high frequency filters are used. Thus, there are filters whose resonators are line sections, whereas in others these components are in wave guide form. The most commonly used filters with resonators in a TEM line are interdigitated filters with quarter wave resonators and comb filters with resonators, charged by localized capacitive elements forming obstacles. Resonator filters in wave guides are differentiated according to their operating mode. When they operate in the propagation mode, i.e. above the cut-off frequency of the guide, the most commonly used is the half-wave resonator filter of the series type coupled by inductance. When they operate in the evanescent mode, i.e. at a frequency below the cut-off frequency of the guide they are constructed as parallel resonators coupled by admittance inverters and they then have localized capacitive obstacles.
These filters are tuned by varying the shape of the localized inductive or capacitive obstacles associated with the TEM lines or the wave guides for forming the filter.
The invention specifically relates to filters incorporating localized capacitive obstacles.
The presently used capacitive tuning devices are usually constructed by means of metal plungers which penetrate the guide or line and the capacitance is regulated by varying the plunger penetration. The variation of the inductance of the capacitive element (linked with the variation of the tuning frequency) obtained in this way, as a function of the frequency, is dependent on the physical configuration of said element and of the cross-section of the guide or line. However, in general terms the variation law of the tuning frequency, as a function of the plunger displacement is not linear. Moreover, in filters in the evanescent mode, the dimensions of the guide in the plane orthogonal to the propagation axis are less than the wavelength corresponding to the cut-off frequency of the guide. Therefore the localized capacitance necessary for obtaining tuning, which increases as the operating frequency decreases, must be located in a smaller space. The capacitance obtained by means of two facing plungers is increased by reducing the cap separating them. However, beyond a certain limit this gap is too small to be accurately adjusted and in addition temperature variations, due to the expansion of metals caused by them, can create very significant relative variations of this gap. The capacitance can then be increased by increasing the facing surfaces. However, this solution is not always practicable by increasing only the diameter of the plungers, because the volume of said plungers is limited through the small dimensions of the evanescent mode-type filters. Finally, as externally the geometry of the plungers in the guide varies as a function of the capacitance required, linked with the tuning frequency, the change in the tuning frequency of the resonator assembly also affects the coupling of said resonator with adjacent components, adjacent resonators or filter inputs. Coupling variations lead to significant variations in the pass band of the filter and to a deterioration in the input and output impedences of this filter as a function of the tuning frequency.